Reactive molecular dynamics simulations of an excess proton in polyethylene glycol-water solutions

We investigate the effects of both triethylene glycol (TEG) and polyethylene glycol (PEG) on the structure and transport of an excess proton (H+) in aqueous solution. A new reactive force field (ReaxFF) parameterisation is carried out to better describe proton transfer of H+ with PEG polymers in aqueous solution via training against density functional theory (DFT) calculations. Using this ReaxFF parameterisation, reactive molecular dynamics simulations are carried out for three different systems containing H+: bulk water, aqueous TEG, and aqueous PEG with a single, oriented polymer chain. The H+ diffusivity is suppressed due to the presence of TEG and enhanced in the oriented chain PEG system, particularly along the axis parallel to the PEG chain, relative to bulk water. The enhancement in proton mobility along the chain axis is mainly due to the structural, rather than vehicular, component. From free energy calculations for H+ transfer, the increase in proton mobility is not due to a reduction in activation energy. From pair distribution function analysis of the local structure for water and hydronium we hypothesise the increased mobility is due to an entropic effect, specifically a favourable hydrogen bond network for long range proton transport due to the oriented PEG chain.